Method and apparatus for drying lumber



Get. 23, 1945. H. J. LUTH E-r m.

METHOD AND APPARATUS FOR DRYING LUMBER Filed Aug. 2, 194s` Qin Patented Oct; 23, 1945 i UNITED METHOD AN APPARATUS Fon nume A Luxueus Harold J. Luth and sydney n. xmpmck,

Muskegon, Mich., assignors to The Brunswick- Balke-Collcnder Company, Chicago, Ill., a corporation of Delaware Application August 2, 1943, Serial No. 496,994

11 claims.

' This invention relates to the drying of lumber.

One object of the invention is to avoid the loss of time incident to the relatively slow process oi kiln drying. Another object is to prevent internal rstrains in the cellularst'ructure ol? the wood which result in checking. honeycomblns and other defects inthe dried lumber. lt is also an object to provide a process in which the "block or billet is heated to a substantially uniform temperature throughout. More specically, we haveV developed a process which pro# duces a high frequency electrostatic field in the body of the wood for heating it and which applies suction or partial vacuum for withdrawing the vapor in the direction of the grain of the wood and not otherwise, to prevent the undesirable eiects heretofore experienced with other Processes.

It is common knowledge in the art oi Vdrying lumber` that as the thickness of the lumber in. creases, resulting in large cross-sectional areas,

"the drying problembecomes complicated.v Because of the low thermo-conductivity of wood, it is very dimcult to uniformly heat through the entire mass, or ii uniform lheating is accoinu pllshed, it is not possible to control the radiation of heat through the mass of lumber to Volotain uniform contraction oi the wood cells during the drying process. Therefore, during the conventional methods of kiln drying lumber it is necessary to start at lowV temperatures with a high relative humidity., for example llo-120 l. at 'Sil-9o relative `luimidity.` Thisis done to allow the moisture of the innermost parts of the block to gradually work its way out without excessively drying the outside of the lumber. Ii this outside portion is dried at such a rate that the moisture from Ythe inside cannot become uniformly distributed 'to the outer shell, stresses are set up in the timber, resulting in checks, honeycombing, and various drying defects.

Those familiar with the lumber art' recognize that in the growth of the .tree there are radialA lines extending from the center outwardly toward the bark and when billets are cut' from the logs some of the cuts intersect these lines. The resulting billets are not uniform in structure, and as moisture leaves the billets the shrinkage is greater at right angles to the radial lines than it is parallel to the radial lines, and it is these differential shrinkages that must be carefully considered inthe Adrying operation.

It is not unusual in drying large billets, for

example billets 'of cross-sectional areas rumiins lil dit'

from 20-25 square inches, to spend as much aS 60 days in the kiln drying operation, starting, as mentioned before, at a. low temperature and `a high humidity, gradually raising the temperatures and lowering the humidity until the desired moisture content is obtained. By taking samples from the kilns it is impossible to determine whether or not the drying rate on the outside of the billet is compatible with the drying on the inside, and vin the event the outer shell is drying toc rapidly, the stresses 'set up are reu' lieved by introducing wet steam into the kiln to relieve'the strains.

With the advent of the use of high frequency oscillators for the generation of heat, experiments were made in the application oi this method to the drying of lumber. llt mst-results were disappointing. This can be laid to 'the fact that high frequency or heatronic heating affects the article in the field through its entire mass, and,`

as a result the temperature tends to build up in the center of a wood bloch more rapidly than on the outside edge for the same reason that in kiln Y drying the reverse takesplace, the reason being the insulating property of the wood. When wood is dried by this method the honeycombing and strains all reverse themselves as compared to the effects oi iriln drying at too great a rate.

We have discovered a means of uniformly removing moisture from large billets withoutsettinsl up these strains. To do this we control the temperature in the billet Vwhile subjecting it to a high frequency eld and this control is accomnlishecl by means of a high. vacuum applied to the billet. By this method the temperature of connect the end of the sach to a vacuum pump and reduce the absolute pressure down to the order of 100. millimeters ol mercury or less. The degree of pressure in terms of absolute units can, of course, vary. We have obtained satisfactory results operating fromBD-l millimeters 'and can speed up the operation by still further lowering the absolute pressure. Y 5

. to perform vthe drying process With such a vprocess there is also atmospheric pressure exerted through the exible wall of the sack on the longitudinal surfaces of the block or billet being dried, roughly 13-14 pounds per square inch; this assists in the collapsing` of the cells and also minimizes the tendency toward `the development of stresses.

By way of illustration the drawing shows. somewhat diagrammatically, apparatus adapted in laccordance with this invention.

In the drawing: f

ascuas I the bottle le t@ a suction creating pump la of any suitable construction. It will be understood that the application of partial vacuum to the porous structure-of the Fig. 1 is a diagrammatic representation of such apparatus in which suction is applied only to one end of the block or billet and vapor .is drawn from the billet through that end only; this arrangement includes meansfor condensing the vapor.

Fig. 2 is a diagram representing a similar apl paratus but with provision for applying a partial vacuum to both ends of the .billet simultaneously and thus drawing oir the vaporized sap or moisture from both ends at the same time.

' Fig. 3 isa diagram of an apparatus by which suction is applied to one end of the billet while the opposite end is left open so that air can be drawn through the billet to aid in carrying oil the moisture.

`While the drawing illustrates certain preferred forms of apparatus and while certain preferred methods are described herein, it is to be under-V stood that the inventionis notlimited to the particular form and arrangement of apparatus herein shown or to the specinc procedure disclosed, it being contemplated that various changes may be made by those skilled in the art without departing. from the spiritand scope oi' the invention as set forth in the appended claims.

Referring to the drawing, there is indicated at I suitable means for supplying high frequency alternating current, includingV an oscillator and other electrical apparatus properly connected and Varrangedv in a circuit for this purpose in accordance with well hewn principles. Leading from the generatorY I there vare conductors 2 and 3 extending respectively to radiating plates or electrodes 4 and 5 shown spaced apart'at opposite means for observing the temperature' is shown installed in the outlet pipe II, comprising a thermocouple indicated at I9 connected to a potentiometer 20 for registering 'the temperature of the vapor issuingfrom the billet 6. We also prefer to 'include a suitable gauge 2I applied to the suction line. as by connection into the bottle I6, so that the suction created by the vacuum.

pump I8 may be constantly observed and controlled in accordance with the best practice determined by experience.

With the apparatus set up as shown in Fig. l, the open end of the rubber sack 1 is tightly connected to the vacuum pump.|8 with the conf. denser i3 disposed between-the sack and the pump and with the collectioibottle I8 in the -line between the condenser andthe pump. The

vtwo radiating plates or electrodes 4' and 5 are placed at opposite sides of th'e billet and slightly spaced therefrom. lIt has been found satisfactory to employ a one kilowatt high frequency oscillator.- at I, having an output of approximately i300 B. t. u.s per hour and operating on 1200 cycles.

The -vacuum is ambient temperature conditions at the start are y approximately 80 F. lAt thevend of 30 minutes sides of the block or billet 6 'which is to be treated.

The block of wood 6 is herein shown as a rough turned billet of the type from which bowling pins are madeand may be understood to be a green undried billetA of maple wood approximately 5 inches in diameter at the largest portion and about 18 inches inlength. The billet Vis enclosed in a snugly fitting rubber sack 'I having walls about one-eighth of an inch thick which substantially seal the surfaces of the billet which they cover. y

In the form shown in Fig. 1 the sack 'I has an end Aw'all 'Ia tting against one end of fthe 450 block or billet 8. butat the other end the sack is closed by a plug 8 around which its side wall may be' secured in air-tight'relationas by a clamping ring 9. In this gureof the drawing we have included means employed in our initial experiments for observing and measuring the actual quantity'of moisture removed from the the-block l. The plug 8 has a-bore l'into which there is connected an outlet pipe II pro'- vlded with a control valve I2 and leading to a vapor condenser comprising a chamber I3l with aoooling water coil I4 extending through it. A delivery'pipe II leads from`\.the Acondenser chamber I3 to'a collector bottle or equivalent receiver It and a vsuction pipe vIl leads Vfrom the top1 of' the temperature within the block will have arisen to 120 F., and the ilrst few drops of distillatewill be obtained; and at the end of 30 minutes approximately 100 cc. of liquid' shouldbe distilled and the temperature will have reached approximately 132 F. In the next 310minutes the temperature should risel to approximately 137 F. and 200 cc. of distillate is obtained, and in the next 70 minutes the final temperature will have risen to about 148 F. and a total of 500 cc. or about 1 pound 3 ounces of moisture will be distilled oil and collected at I6. It has been noted that the temperature rises of its own ac-i cord toward-the end of the distillation.v This can be controlled by varying the output `ofthe oscillator and by altering the degree of vacuum applied to the block. v

The economy that caribe effected by this method of drying is quite obvious. By roush- I calculation it requires approximately it-2% kilowatts to effect a complete drying and the power consumption is in proportion to the reduction in moisture.' It hasv also'been-found that.

when the wood block is unusually wet it is not necessary to immediately apply the highfref quencyheatins. In cases where we have taken timber fresh from the woods with.' moisture con- .tent as high as (i5-'75%, in about' one hour or less we have distilled ot! underhlsh vacuum enough water under ambient conditims to bring the moisture content down to about 2li-22%; and

tneaecqnammggofmmmg.

started up by reducing the pressure to about millimeters absolute pressure and the oscillator is placed in operation. Thel im therefore' necessary onlyto apply neatiuriiia .was approximately 1 pound 3 ounces.

With the apparatus shown in Fig. 2 the mode of operation will be quite similar, but the time posite ends of the tubular sheath 30, and these heads are iltted .with outlet pipes 33 and 3l respectively, both connected to the delivery tube 35 which leads to the vacuum pump 3B. With this arrangement and with a. vacuumI pump of suitable capacity the vaporized liquid may be with- -of said grain. and simultaneously heating the.

drawn simultaneously from both ends of the billet 6. l

For commercial purposes there will be no need of actually condensing the vapor after it has been once withdrawn from the billet, and the cony denser and collector bottle are therefore dispensed with Similarly, with the apparatus shown in Fig. 1, the condenser I3 and collector I6 may be omitted in commercial practice.

It will be understood thatthe foregoing description illustrates the principles of the invention although the mechanical setup for performing it may be varied considerably, as by changing the shape and location of the electrodes and the particular means for applying suction to the billet 8. For example, we have been able to somewhat speed up the drying time by means of a partial vacuum and a current of air pulled in a direction parallel to the grain of the block by leaving one end of the block open and depending upon resistance to the flow of air through the -billet to build up the low pressure therein. 3

shows an apparatus-arranged to operate in this way. The rubber sheath is open at one end, leaving the end grain of the billet exposed to the air. 'I'he other end 'of the sheath is provided with a head 4i from which an outlet pipe 42 leadsdirectly to the vacuum pump 43. By this method more economy is effected in the use of comparatively dry air entering one end of the the grain in an air-tight encasement in contact.

billet and picking up moisture on its way throughq and thus speeding up the operation. Obviously, however, this requires a greater power input into the source of vacuum in order toV remove the noncondensible gases from the free air flowing through the billet, and therefore this method may not offer any substantial advantage over that already described. In practice we have preferred to operate with the sack closed to the outer atmosphere as in Figs. 1 and 2, since with this arrangement .it is possbleto more accurately control the operation. f

block internally to expedite vaporization of the moisture therein.

3. The method of drying a block of wood which includes encasing it in a snug-iitting exible sheath of air-tight material, applying suction to at least one end surfaceof the block which extends across the grain thereof, and simultaneouslyheating the block internally to expedite vapori- 4ation of the moisture therein.

4. The method of drying a block of wood 'as `cleaned in claim 3,111 which the internal heating is accomplished by 'disposing the encased block between spaced electrical conductive elements and creating a high frequency alternating poten tial difference between said elements.

5. The method of drying a block of wood which includes encasing it in a snug-iitting flexible sheath of air-tight material, and applying suction to both ends of the block simultaneously while heating the block internally 4by means `of a high frequency electrostatic field. Y

6. .The method of drying a block of wood which includes enclosing the surfaces thereof which extend substantially parallel ito the grain in an airtight encasement in contact' with said surfaces, applying suction to an end surface of the block extending across the grain, and simultaneously subjecting the block to a high frequency electrostatic field.

7. The method of drying lumber which includes enclosing the surfaces of the lumber parallel to with said surfaces, applying reduced pressure to one end of the lumber and thereby dnawing air vthrough it in the direction of the grain, and

9. The method of drying a block of wood which includes sealing the entire surface thereof which extends substantially parallel to the grain and We have been successful in-drying billets ap- Y proximately 51/2" in diameter and 18" long in about 2 hours and'15 minutes. The billets originallyweighed '1 pounds 4 ounces and when dried weighed about 6 pounds 1 ounce and had a final` moisture content of approximately 7%. The original block -contained approximately 23% moisture. The actual amount of water removed The 11nished billets showed perfect drying conditions free from honeycombing, case hardening, and had no evidence of checks or strains when thin sections were cut parallel to the radial lines, split apart andrematched together.

We claim:

1. The method of drying a block of wood which includes substantially sealing the entire surface thereof which extends in the general direction of the grain, internally heating the block and simultaneously extractingfvapor from .the bloclr applying suction to an end surface of theA block extending across Vthe grain.

l0. 'Ihe method of drying a block of wood which includes sealing all surfaces of the block except 4one which extends across the grain, and applying suction to said one surface.V

11. The method of drying a block of wood which includes sealing all the surfaces thereof -which extend substantially parallel to the grain and applying suction simultaneously to opposite end lsurfaces extending across the grain.

12. The method of drying a block of wood .the suction producing means to closing the ends of said sheath, an outlet leading from one of such closing means, and suction producing means, with a conduit directly connecting the closing means.

14, Apparatus for drying a block or billet of,

wood including a sheath impervious to air fitted snugly over the `surfaces of the block which extend in the general direction of its grain, means closing the ends of said sheath with outlets leadsaid outlet of ing from said end closing means, and suction f suction producing means to said outlet of thel closing means', the other end of the sheath being open, exposing to the air an end surface of the block which extends across the grain. l

16. Apparatus for drying a block or billet oi wood including a sheath of ilexible material inrpervious to air and adapted to fit snugly over the surfaces of the block which extend in the general direction of its grain, means closing the ends of said sheath, an outlet conduit leading from at least one of such closing means, suction producing means, with a conduit directly connecting said suction producing means to said outlet of the closing means, a pair of electrically conductive elements spaced from said sheath at opposite sides thereof, 'and a high frequency alternating current generator connected to said elements.

17. Apparatus for drying a block orV billet of Wood yincluding a. sheath of rubber adapted to ilt snugly over the surfaces of the block which extend in the general direction of the grain,means closing the ends of said sheath, an outlet in one of such end closing means, a. vacuum pump, a.

. conduit directly connectingsaid pump with said outlet, a pair of electrically conductive radiating plates `spaced from saidv sheath at opposite sides l thereof, and a high frequency alternating current source connected to said plates.

HAROLD .LLUTEL v 4 SYDNEY R. KRUPNIGK. 

